Diesel versus biodiesel, which is better for the environment? The answer to the question seems to have an obvious answer. The names alone indicate which is better for the environment, right? The prefix “bio” implies that biodiesel is inherently clean. Not only does the name imply biodiesel is clean, so does the marketing of biodiesel. Generally, the marketing of biodiesel includes claims about the cleanliness of biodiesel, especially its cleanliness in relation to petrodiesel. 

But, the name used to market a fuel has no impact on the emissions a fuel produces. Nor do marketing campaigns always provide consumers with the information necessary to make an informed decision. Consequently, the name biodiesel and its marketing complicate the issue of the environmental cleanliness of biodiesel. 

The reality is, biodiesel may not be clean at all. Biodiesel may be more damaging to the environment than petrodiesel, far more. There is a strong possibility diesel is the superior fuel in the diesel vs biodiesel debate.

Cleanliness of Biodiesel: Beyond Carbon Dioxide and Carbon Footprint

Determining the carbon footprint of a fuel requires more than a simple analysis of combustion emissions. It certainly requires more than a per-gallon breakdown of carbon dioxide emissions. Even if a transparent analysis of carbon dioxide emissions occurs, it does not tell the whole story. A fuel’s carbon footprint does not account for the other emissions a fuel’s combustion produces. Carbon footprint only accounts for carbon dioxide. 

And, emissions do not tell the whole story either. The environmental impact created during the production of a fuel is also a factor. In addition to the carbon footprint of a fuel and the environmental impact of production, other variables determine fuel cleanliness. 

Even a cursory understanding of the emissions produced by biodiesel make it clear biodiesel is not as clean as often advertised. It is very likely that biofuel produces more — and worse — emissions than petrodiesel. 

Biofuel Emissions as a Whole

What is not up for debate is the fact that biofuel produces more carbon dioxide than traditional diesel. And not only does biodiesel generate more carbon dioxide, biodiesel produces more nitrogen oxides than does petrodiesel. Furthermore, biodiesel engines are considerably less fuel efficient. And, emissions and energy density do not account for the local and regional devastation that occurs as a result of growing crops for biodiesel. 

Growing crops for biodiesel requires exponentially more land than required for an oil rig. The energy consumed to produce biodiesel is also exponentially greater than that necessary to manufacture traditional diesel. And, the waste products generated from the production of biodiesel is, too, exponentially higher than that left behind from the production of petrodiesel.

Still, measuring the environmental impact of growing the crops is difficult. The impact from growing biodiesel crops is typically specific to the geographic traits and economic characteristics of a given region. 

But the emissions biodiesel produces, versus traditional diesel, is much easier to determine on a global scale. Even a layman can see that biodiesel — any biofuel for that matter — is anything but clean.

The Error of Arguing Emissions Per Unit of Volume: Gallon, Liter, Cubic Foot, Etc.  

Emissions produced per unit of volume is a useless standard by which to compare the emissions of fuels. There is almost no value in an emissions-per-gallon measurement, for example. The emissions a fuel generates per unit of volume is of no value because different fuels produce different amounts of energy per unit of volume. As such, even if a fuel produces more emissions per unit of volume, the fuel may produce far fewer emissions because each gallon produces more work. 

A comparison of gasoline vs. diesel carbon dioxide emissions illustrates the problem with promoting a fuel based on its per unit of volume emissions. Diesel generates slightly more carbon dioxide than gasoline per gallon. But, a diesel engine gets better fuel efficiency. “About 19.6 pounds of CO2 are produced from burning a gallon of gasoline that does not contain fuel ethanol. About 22.4 pounds of CO2 are produced from burning a gallon of diesel fuel,” according to the U.S. Energy Information Administration. 

So, per gallon, diesel produces 12.5 percent more CO2. 

Fuel Consumption of Diesel Engine vs Gasoline Engines

But, according to the U.S. Department of Energy, “diesel fuel contains roughly 10% to 15% more energy than gasoline.” Additionally, diesel engines have far greater thermal efficiency than gasoline engines. A diesel engine converts far more energy into work than a gasoline engine. A gasoline loses between 60 and 80 percent of the energy a fuel produces out the tailpipe or to the air around an engine. 

That means the fuel efficiency of a diesel engine — the “gas” mileage — is far better than that of gasoline engines. “Diesel vehicles can often go about 20% to 35% farther on a gallon of fuel than their gasoline counterparts,” the continues the Department of Energy.

So, per mile, a diesel engine produces between 8 and 27 percent less carbon dioxide than a comparably sized diesel engine.

So, how does biodiesel compare to traditional petrodiesel? The short answer, not well. Not well at all. The reason for the difference between biodiesel and petrodiesel is the hydrocarbons each contains. 

Comparison of Hydrocarbons in Traditional Diesel and Biodiesel

In relation to traditional diesel, the molecular composition of biodiesel is not unique. Just as petrodiesel is, biodiesel is a mixture of hydrocarbons. All fossil fuels and all biofuels are made of hydrocarbons. The difference between biodiesel and petrodiesel are the categories and types of hydrocarbons within each.

Categories and Classes of Hydrocarbons

While there are thousands of hydrocarbons types, there are only two categories of hydrocarbons and four classes. The two categories are saturated and unsaturated hydrocarbons. And both categories of hydrocarbons have two classes. Paraffins and naphthenes are the two saturated hydrocarbon classes. Aromatics and olefins are the two unsaturated hydrocarbon classes.

The mixtures and ratios of the four classes of hydrocarbons determine fuel type.

Given that there are only two categories of hydrocarbons and only two classes within each category, many of the elemental components of traditional diesel and biodiesel are the same. Traditional diesel and biodiesel are mixtures of different ratios of the same hydrocarbons. 

So, the answers to two questions determine which fuel is safer for the environment. First, “which of the two fuels contains a higher amount of less polluting hydrocarbons?” And second, “which produces the least amount of emissions per mile — again, not per unit of volume?” 

Hydrocarbons in Traditional Petrodiesel

The four classes of hydrocarbons are alkanes, cycloalkanes, alkenes, and olefins.  But, only three classes of hydrocarbons constitute diesel: alkanes, cycloalkanes, and alkenes.  Olefines do not occur naturally in crude oil. Though hydrocarbons, they are a byproduct of the crude oil refining process. But, olefins are not a byproduct of the diesel refining process. So, diesel has only trace amounts of olefins. 

Alkanes and cycloalkanes are saturated hydrocarbons. They cannot take on more hydrogen or carbon atoms. Nor, can alkanes and cycloalkanes take on any other atoms for that matter. Alkanes and cycloalkanes are extremely stable hydrocarbon molecules. And, both the alkanes and cycloalkanes in diesel have high energy density because they are typically large, long-chain molecules. 

Alkanes are the general term for single-chain, stable hydrocarbons. They contain a single spine of carbon atoms, each carbon atom bonded to three hydrogen atoms. The petroleum industry refers to alkanes as “paraffins.” 

The petroleum industry refers to cycloalkanes as “naphthenes.” Cycloalkanes — naphthenes — have branches. The carbon atoms on the central spine of a cycloalkane may have a bond with more than two carbon atoms. And cycloalkanes loop. One end of a naphthene hydrocarbon molecule chain spine has a bond with the opposing end.  

Paraffins and naphthenes constitute the large majority of the molecules in diesel. “The aliphatic alkanes (paraffins) and cycloalkanes (naphthenes) are hydrogen saturated and comprise approximately 80-90% of the fuel oils. Aromatics (e.g., benzene) and olefins (e.g., styrene and indene) compose 10-20% and 1%, respectively, of [diesel].” according to the U.S. Health and Human Services Department.” 

Advantages of Alkane Paraffins and Cycloalkane Naphthenes

Traditional fossil fuel is arguably the cleanest, highest-energy fossil fuel. It is almost unarguably the cleanest, highest-energy transport fuel. The reason, alkane paraffins, and cycloalkane naphthenes. Paraffins and naphthenes are saturated hydrocarbons. Again, they are complete hydrocarbons without room for the addition of more hydrogen or carbon atoms. That makes diesel extremely stable. 

And, in the world of hydrocarbons, paraffins and naphthenes typically have higher energy density than aromatics and olefins because — generally — they are larger, longer chains. 

Biodiesel is quite different, almost exactly the opposite in fact. 

Hydrocarbons in Biodiesel

Biodiesel is made up almost entirely of unsaturated hydrocarbons. “Biodiesel molecules consist almost entirely of chemicals called fatty acid methyl esters (FAME), which contain unsaturated ‘olefin’ components. Low-sulfur petroleum diesel, on the other hand, consists of about 95 percent saturated hydrocarbons and 5 percent aromatic compounds,” according to PennStateExtension. 

What is Fatty Acid Methyl Esters

To produce FAMEs, from plant stock, it is necessary to combine the fatty acid esters from the plant material with alcohol. A process called transesterification replaces the ester atoms in fatty acid ester molecules with alcohol molecules. The result is fatty acid methyl esters. FAMES are the hydrocarbons in biofuel that ignite/combust/burn. 

Fuel Density of and Emissions from Petrodiesel Vs Fatty Acid Methyl Esters

Per mile — or any distance for that matter — petrodiesel and biodiesel are some of the cleanest fossil fuels available. For one, neither one produces carbon monoxide at levels beyond trace amounts. For two, both traditional diesel and biodiesel produce lower amounts of carbon dioxide than gasoline and alternative gasoline fuels. 

However, emissions of concern with respect to petrodiesel and biodiesel include sulfur oxides and nitrogen oxides. 

Biodiesel Magazine explains:

“An overwhelming number of studies show that using neat biodiesel and biodiesel blends in compression ignition engines slightly increase the generation of nitric oxide and nitrogen dioxide-any combination of which constitutes oxides of nitrogen (NOx). Without dwelling on technicalities, NOx increases can range anywhere from less than 1 percent to 15 percent higher than petrodiesel, with a long list of “depending on” factors.”

The implications of biodiesel being almost entirely fatty acid methyl esters are three-fold. First, biodiesel is low in energy density in relation to traditional diesel. The energy density of biodiesel is between 8 and 10 percent lower than low-sulfur diesel. The difference is even greater between biofuel and regular diesel. Because biodiesel has a low energy density, that means more diesel is necessary to move a vehicle a given distance. 

Nitrogen oxides are another issue with biodiesel. And finally, the fact that FAMEs contain unsaturated olefin components makes biodiesel’s cleanliness extremely questionable. 

Nitrogen Oxides Emissions from Biodiesel

Nitrogen oxide results from the combustion of both biofuel and petrodiesel. However, the levels of nitrogen oxide produced from the combustion of traditional diesel are significantly lower than that biodiesel produces. During the refinement of petrodiesel, it is possible to remove all but trace amounts of the nitrogen. Therefore, refined diesel only produces trace amounts of nitrogen oxides.

Biodiesel, on the other hand, produces large quantities of nitrogen oxides. There are seven types of nitrogen oxides. Nitrogen dioxide is one of the most toxic and dangerous emissions produced by combustion engines. Nitrogen oxide — laughing gas — is one of the most dangerous greenhouse gases produced by combustion engines. 

Toxicity of Nitrogen Dioxide

According to the EPA, nitrogen dioxide is a major contributor to acid rain, the other being sulfur oxides. Additionally, nitrogen dioxide causes nutrient pollution. “Too much nitrogen and phosphorus in the water causes algae to grow faster than ecosystems can handle. Significant increases in algae harm water quality, food resources, and habitats, and decrease the oxygen that fish and other aquatic life need to survive. Large growths of algae are called algal blooms and they can severely reduce or eliminate oxygen in the water, leading to illnesses in fish and the death of large numbers of fish.

While nitrogen dioxide does serious damage to the environment, its effects are less devastating than those from nitrogen oxide. 

Toxicity of Nitrogen Oxide

One of the most dangerous greenhouse gases produced by humans, nitrogen oxide has a global warming potential exponentially greater than carbon dioxide. “Nitrous Oxide (N2O) has a GWP 265–298 times that of CO2 for a 100-year timescale. N2O emitted today remains in the atmosphere for more than 100 years, on average,” explains the EPA.

“Biodiesel’s increased NOx emissions continue to be the mainstay of critics’ arguments against the widespread use of the renewable fuel,” explains Biodiesel Magazine. But, no one is quite sure why biofuel produces such an excess of nitrous oxide. “What is it about biodiesel that tends to increase NOx? Cetane numbers are higher, which should reduce NOx, and aromatic hydrocarbons are lower. Some say that the oxygen content in biodiesel is cause for higher NOx emissions. ‘One reason for increased NOx could be the molecular weight of the methyl ester,’ said M. Lie Ken Jie of the University of Hong Kong.”

Irrespective of the reason, biodiesel produces extremely high levels of nitrogen oxides. Because of advanced refining technologies, petrodiesel does not. 

The Truth About Biodiesel

Many of the effects of producing and combusting biodiesel are still unknown. What is known, however, is that biodiesel is not nearly as clean as once thought. Biodiesel produces high quantities of carbon dioxide throughout its lifecycle. Additionally, biodiesel produces large quantities of nitrous oxides.

It’s worth taking the time to step back and reconsider the cleanliness of biodiesel.